1. Field of the Invention
The present invention relates to a solid-state imaging apparatus with an electronic shutter function and, in particular, to a solid-state imaging apparatus which discharges a carrier toward a substrate.
2. Description of the Related Art
Hitherto, a solid-state imaging apparatus has used a so-called a vertical overflow drain (VOFD) in which a voltage is applied to a semiconductor substrate to discharge a carrier of a photoelectric conversion element to control accumulation time, thereby realizing an electronic shutter function. Particularly, the function is often used in a CCD solid-state imaging apparatus.
Japanese Patent Application Laid-Open No. 2005-166731 proposes a solid-state imaging apparatus which causes a MOS solid-state imaging apparatus to operate a VOFD function to realize an electronic shutter function.
A structure shown in Japanese Patent Application Laid-Open No. 2005-166731 leaves room for improvement in the following respects.
Applying a high voltage to an N-type semiconductor substrate to realize the VOFD function significantly varies a potential in a buried layer due to a capacitive coupling between the substrate and a P-type buried layer acting as a potential barrier. This may significantly vary a potential in a P-type semiconductor region electrically connected to the buried layer. Particularly, if a pixel has a carrier accumulating region for temporarily holding a signal carrier from a photoelectric conversion region, a P-type semiconductor region forming a potential barrier is provided in the vicinity of the carrier accumulating region to effectively accumulate the carriers to significantly vary a potential in the P-type semiconductor region. As a result, the carriers accumulated in a carrier accumulation region leak toward the substrate to vary the amount of accumulated carriers, which may degrade image quality.
Variation in potential in the P-type semiconductor region forming the potential barrier can result also from variation in potential of a source and a drain in the operation of each MOS transistor. This is because a potential of a P-type well being a back gate of the MOS transistor capacitively coupled with the source or the drain varies to vary a potential in the P-type semiconductor region forming the potential barrier electrically connected to the well.
The present invention has been made in view of the above problem and an object of the present invention is to moderate variation in the amount of accumulated carriers when the VOFD function is used to realize an electronic shutter function, thereby performing a high image-quality photographing.